High performance creep resistant magnesium alloys

ABSTRACT

The invention provides a magnesium based alloy consisting of at least 94.8% magnesium, 2.5-4.6% neodymium, 0.05-0.40% yttrium, and 0.03-0.65% zirconium, exhibiting good castability, high strength, high corrosion resistance and high creep resistance even at high temperatures. The alloy is suitable for high pressure die casting, sand casting, investment casting, permanent mold casting, twin roll casting, or direct chill casting.

FIELD OF THE INVENTION

The present invention relates to creep-resistant magnesium-based alloysfor applications at high temperatures which exhibit good castability,particularly suitable for high-pressure die casting, but advantageouslyused also in a processes comprising sand casting, investment casting,permanent mold casting, as well as direct chill casting or twin-rollcasting.

BACKGROUND OF THE INVENTION

The magnesium industry is experiencing dramatic growth, in part due tothe demands of the transportation industry to improve fuel economy andemissions. In addition, a great progress in weight reduction has beenmade in consumer applications of magnesium alloys such as power handtools, lawn and garden equipment, electronic and optical equipment, etc.In order to significantly expand the above applications, new advancedalloys are required.

High pressure die casting (HPDC) is the dominant form of casting due toits productivity and suitability for mass production. Currently, mostcommon and new magnesium alloys that are being used for HPDC process areAl containing alloys. However, these alloys cannot serve at temperatureshigher than 150-170° C. under high stresses of 60-100 MPa. U.S. Pat. No.6,193,817 describes magnesium-based alloys containing 0.1-2.0 wt % Zn,2.1-5.0 wt % RE elements (Ce based mischmetal) up to 0.4 wt % of acombination of at least two elements chosen from the group consisting ofZr, Hf and Ti, and optionally up to 0.5 wt % Mn and up to 0.5 wt % Ca.High-pressure die casting of the alloys results in low strength (TYS=120MPa, UTS=165 MPa) and elongation (E=2%). EP 1866452 discloses magnesiumbased alloys containing 1.5-4.0% RE elements, 0.3-0.8% Zn, 0.02-0.1% Al,4-25 ppm Be and optionally up to 0.2% Zr, 0.3% Mn, 0.5% Y, and 0.1% Ca.The alloys, under die cast conditions, exhibit low tensile strength(TYS=130 MPa, UTS=160 MPa) and elongation (E=1-3%).

WO 2009/086585 relates to magnesium based alloys containing 2-5% REelements (primarily La and Ce, wherein La content is higher than Cecontent) and 0.2-0.8% Zn. In addition, the alloys contain optionally Y,Gd, Zr, Mn, Ca, and Be. These alloys are also designated forhigh-pressure die casting but exhibit very low values of elongation,TYS, and UTS.

SU 1,360,223 discloses Mg-based alloys containing 0.1-2.5% Zn, 0.3-1.0%Zr, 0.8-4.5% Nd, 0.5-5.0% Y, 0.8-4.5% Gd, and 0.01-0.05% Mn. The alloysare intended for sand casting process and exhibit optimal propertiesafter full T6 treatment.

U.S. Pat. No. 4,116,731 describes heat treated and aged magnesium basedalloys containing 0.8-6.0 wt % Y, 0.5-4.0 wt % Nd, 0.1-2.2 wt % Zn,0.3-1.1 wt % Zr, up to 0.05% Cu, and up to 0.2% Mn. Due to relativelywide concentration ranges claimed by the above patent, the alloysexhibit very diverse properties; in addition, they are designated onlyfor sand casting process

EP 1329530 discloses magnesium-based casting alloys containing 0.2-0.8wt % Zn, 0.2-0.8 wt % Zr, 2.7-3.3 wt % Nd, 0.0-2.6 wt % Y, and0.03-0.25% Ca. The alloys exhibit high strength and high creepresistance after gravity casting and after full T6 heat treatment, aswell as after extrusion and forging. However, the HPDC is not addressed.

CN 1752251 describes magnesium alloys containing 0.35-0.8 wt % Zr,2.5-3.6 wt % Nd, 0.0-0.4 wt % Zn, 0.0-0.5 wt % Ca, and 0.0-0.02 wt %impurities. The alloys are prepared by a two-stage process including astep of preparing intermediate master alloys Mg—Nd, Mg—Ca, and Mg—Zr,and a step of alloying said master alloys by Nd, Ca, and Zr. Thecomplexity of the technology significantly increases the cost of thefinal alloy product.

EP 1641954 describes creep resistant alloys containing 2.0-4.5% Nd,0.2-1.0% Zr, 0.2%-7.0% HRE (Heavy Rare Earth Elements of atomic numbers62-71), optionally up to 0.4% of other RE elements, up to 0.5% Y, up to1.3% Zn, up to 0.5% Mn, and up to 0.4% Hf or Ti. The alloys are mainlydesignated for sand casting and, in addition, they are expensive due tothe use of heavy rare earth elements, such as Gd in amounts of 1.0-1.6%.

US 2009/0081313 relates to biodegradable magnesium alloys containing1.5-5.0% Nd, 0.1-4.0% Y, 0.1-2.0% Ca, and 0.1-1.0% Zr. The alloys aredesignated for manufacturing medical implants by extrusion. The high Cacontent results in increased porosity, embrittlement and hot cracking inHPDC processes.

WO 2010/038016 relates to magnesium alloys containing 2.0-4.0% Y,0.5-4.0% Nd, 0.05-1.0% Zr, 0.0-5.5% Gd, 0.0-5.5% Dy, 0-5.5% Er, 0.0-0.2%Yb, and 0.0-0.04% Sm. In addition, the total content of Gd, Dy and Er isin the range of 0.3-12 wt. %. The alloy is dedicated for sand casting,and it can also be used as a wrought alloy. The alloy is unsuitable forHPDC process. Furthermore, the high content of heavy rare earth elementsleads to high cost of these alloys.

WO 2011/117628 describes magnesium alloys containing 0.0-10.0% Y,0.0-5.0% Nd, 0.00-1.2% Zr, 0.0-0.3% Gd, and 0.0-0.2% Sm, wherein thetotal content of Ho, Lu, Tm and Tb is in the range of 0.5-5.5%. Thealloy is dedicated for manufacturing medical implants. Due to very wideconcentration ranges of Y, Nd, and heavy rare earth elements Ho, Lu andTm, the alloys exhibit very diverse properties. The alloys are notsuitable for HPDC process and are expensive.

It is therefore an object of this invention to provide magnesium alloyssuitable for high pressure die casting (HPDC) applications.

It a further object of this invention to provide magnesium-based alloysallowing crack-free castings at HPDC applications.

It is also an object of this invention to provide magnesium-based alloyshaving a superior combination of strength and ductility, as well ascapability to operate at a temperature of 200° C. for a long time.

It is another object of the present invention to provide alloys whichare also suitable for sand casting, investment casting, and permanentmold casting, and which exhibit excellent combination of castability,creep performance, and corrosion resistance.

It is a still further object of this invention to provide alloys whichare also suitable for direct chill casting and twin roll casting withsubsequent plastic forming operations such as rolling, forging andextrusion.

It is still another object of this invention to provide alloys whichexhibit the aforesaid behavior and properties, and have an affordablecost.

Other objects and advantages of the present invention will appear as thedescription proceeds.

SUMMARY OF THE INVENTION

The invention provides a lightweight alloy for high-pressure die casting(HPDC) process, consisting of at least 94.8 wt % magnesium, 2.5 to 4.6wt % neodymium, 0.05 to 0.40 wt % yttrium, 0.03 to 0.65 wt. % zirconium,and incidental impurities. In one embodiment, the alloy according to theinvention further contains up to 0.02 wt % calcium. The alloy accordingto the invention comprises essentially no heavy rare earth (HRE)elements with the atomic number from 61 to 70. The alloy according tothe invention comprises essentially no cerium, lanthanum, andpraseodymium. The alloy according to the invention comprisingessentially no zinc. In one embodiment, the alloy according to theinvention contains Nd and Y in an amount higher than 4.3 wt. %. Saidincidental impurities usually comprise Si, Fe, Cu, and Ni in an amountof up to 0.02 wt %. The lightweight alloy according to the invention issuitable for prolonged operations at high temperatures of up to 200° C.The alloys for HPDC and other applications according to the inventionexhibit superior casting properties, high strength, high creepresistance, high corrosion resistance, and the articles manufacturedfrom the alloys show superior performance at high temperatures. Thealloy according to the invention is usable for high pressure diescasting (HPDC), but it may be also used for a process selected from thegroup consisting of sand casting, investment casting, and permanent moldcasting. The alloy according to the invention are also usable for aprocess comprising either twin roll casting with subsequent rolling, ordirect chill casting with subsequent forging, extrusion or rolling.

In a preferred embodiment of the invention, the lightweight alloy isadvantageously used for HPDC. In one embodiment, the alloy suitable forHPDC contains 2.8 to 4.3 wt % Nd, 0.06 to 0.25 wt % Y, 0.05 to 0.4 wt %Zr, and 0.0 to 0.02 wt % Ca. In a preferred embodiment of the invention,the alloy used for HPDC exhibits a tensile yield strength (TYS) at 200°C. of at least 153 MPa, a compression yield strength (CYS) at 200° C. ofat least 152 MPa, a minimum creep rate of not more than 1.5×10⁻¹⁰/s at200° C. under stress of 100 MPa, and a corrosion rate of not more than2.65 mpy. When measured in relative units characterizing oxidationresistance, fluidity, and dies sticking, the alloy according to theinvention preferably exhibits a castability of at least 96%.

The invention relates to an article cast of the alloy containing 2.8 to4.6 wt % Nd, 0.06 to 0.25 wt % Y, 0.05 to 0.4 wt % Zr, and 0.0 to 0.02wt % Ca, the article exhibiting a superior combination of strength andductility after T5 treatment which includes direct aging at 150-250° C.for 1-10 h. In one embodiment, said article exhibits a superiorcombination of strength and ductility after T5 treatment which includesdirect aging at 175-225° C. for 1-6 h.

The alloy according to the invention is also suitable for sand casting,investment casting, permanent mold casting, and low pressuremodifications thereof; in one embodiment, the alloy contains 2.7 to 3.4wt % Nd, 0.15 to 0.40 wt % Y, 0.3 to 0.6 wt % Zr, and 0.0 to 0.02 wt %Ca. The invention relates to an article cast of said alloy, the articleexhibiting a superior combination of performance properties after fullT6 heat treatment comprising solid solution heat treatment at 520-560°C. for 1 to 16 hrs, followed by cooling in a quenching medium and bysubsequent aging at 200-270° C. for 1 to 16 h. In one embodiment, saidarticle exhibits a superior combination of performance properties afterfull T6 heat treatment comprising solid solution heat treatment at535-545° C. for 3 to 5 hrs, followed by cooling in a quenching mediumand by subsequent aging at 225-250° C. for 3 to 6 h.

The alloy according to the invention may be advantageously used forforging, extrusion, and rolling; in one embodiment, the alloy contains2.8 to 3.8 wt % Nd, 0.20 to 0.40 wt % Y, 0.35 to 0.60 wt % Zr, and0.0-0.02 wt % Ca. The invention relates to an article cast in saidalloy, the article exhibiting a superior combination of performanceproperties after T5 heat treatment comprising aging at 200-250° C. for 1to 16 h.

The present invention provides creep-resistant magnesium-based alloysdesignated for applications at temperatures as high as 200-250° C.,which exhibit good castability and low susceptibility to hot tearing,which are strong and are corrosion resistant, and have excellentductility.

The invention provides a process for manufacturing a lightweight alloyfor prolonged operation at high temperatures of up to 200° C.,comprising steps of i) alloying magnesium with neodymium and zirconiumat 765-785° C. under intensive stirring; ii) settling the melt for 20-40minutes to allow iron to settle; iii) adding yttrium, while avoidingintensive stirring to prevent the formation of Y—Fe intermetalliccompounds; iv) optionally adding calcium prior to settling; v) settlingthe molten alloy for 30-60 minutes; and v) casting into desired form;wherein the steps are performed under a protective atmosphere ofCO₂+0.5% HFC134a till solidification; the amount of magnesium in thealloy being at least 94.8 wt %, of neodymium from 2.5 to 4.6 wt %, ofyttrium from 0.05 to 0.40 wt %, of zirconium from 0.03 to 0.65 wt. %,and of calcium 0.00 to 0.02%. The lightweight alloys thus manufacturedare particularly suitable for high pressure die casting, but can beadvantageously employed in sand casting, investment casting, andpermanent mold casting.

The alloys according to the invention contain more than 94 wt %magnesium, from 2.5 to 4.6 wt % neodymium, from 0.05 to 0.40 wt %yttrium, from 0.03 to 0.65 wt % zirconium, optionally calcium up to 0.02wt %, and incidental impurities. The alloys usually contain up to 0.007wt % iron, up to 0.001 wt % nickel, up to 0.003 wt % copper, up to 0.015wt % silicon, and eventually other incidental impurities. The alloys ofthe invention exhibit an excellent combination of high tensile andcompressive yield strength, and high ductility. The great advantage ofnew alloys is related to their high creep rupture stress, creep strengthand low minimum creep rate, as well as low corrosion rate measured underGM 9540 cyclic corrosion test. Thus, the alloys of the present inventioncombine superior performance properties, good castability, andrelatively moderate cost. Articles according to the invention arepreferably subjected to T5 or T6 heat treatments depending on precedingcasting process and plastic forming operations.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other characteristics and advantages of the invention willbe more readily apparent through the following examples, and withreference to the appended drawings, wherein:

FIG. 1. is Table 1, showing chemical compositions of alloys for HPDCaccording to the invention (Examples 1-7) and comparative alloys(Comparative Examples 1-7);

FIG. 2. is Table 2, showing the results of die castability evaluationfor alloys of Table 1;

FIG. 3. is Table 3, showing mechanical and corrosion properties of thealloys of Table 1;

FIG. 4. is Table 4, showing creep properties of the alloys of Table 1;

FIG. 5. is Table 5, showing chemical compositions of alloys for sandcasting according to the invention (Examples 8-14) and comparativealloys (Comparative Examples 8-14);

FIG. 6. is Table 6, showing mechanical and corrosion properties of thealloys of Table 5;

FIG. 7. is Table 7, showing chemical compositions of alloys according tothe invention after forging (Examples 15-18) and comparative alloys(Comparative Examples 15-18);

FIG. 8. is Table 8, showing mechanical properties of the alloys of Table7; and

FIG. 9. is a scheme showing GM 9540 cycling test procedure for thecorrosion evaluation.

DETAILED DESCRIPTION OF THE INVENTION

It was found that certain combinations of elements in magnesium basedalloys comprising neodymium, zirconium, yttrium, and optionally calcium,impart to the alloys superior properties, particularly for high pressuredie casting. These properties include excellent combination of hightensile and compressive properties with high ductility, outstandingcorrosion resistance, and creep properties allowing to achieve servicetemperatures up to 250° C.

The above combination of properties can be realized at high-pressure diecasting, at sand casting, as well as at direct-chill casting or twinroll casting followed by plastic forming processes such as forging,extrusion and rolling.

Magnesium-based alloys of the instant invention contain 2.5 to 4.6 wt %neodymium. It was found by the inventors that if the Nd content is lessthan 2.5 wt %, the alloys have insufficient strength at ambient andelevated, and their creep resistance is not sufficient for serving at250-300° C. temperatures; Nd content higher than 4.6 wt % results in lowductility due to excessive amount of intermetallic compounds which aresources of cracks initiation and propagation. An alloy according to thepresent invention contains 0.05 to 0.40 wt % yttrium. It was found thatyttrium content less than 0.05 wt % makes the alloy prone to oxidationand results in increased susceptibility to burning during molten metalhandling at 700-780° C. On the other hand, increasing the yttriumcontent to more than 0.40 wt % leads to lower ductility, significantlydeteriorated castability, while increasing the alloy cost. Zirconium ismainly used to remove iron in the case of high pressure die castingprocess. In the case of gravity casting (sand casting, investmentcasting, and permanent mold casting) it also serves as a grain refiner.It has been found that 0.03 wt. % of Zr is sufficient to ensure low ironcontent in the alloy, while at least 0.3 wt % of zirconium is requiredfor grain refining. The upper limit for the zirconium content is about0.65 wt % due to its limited solubility in Mg—Nd—Y alloys.

The alloys of the present invention contain substantially no zinc; itwould deteriorate creep resistance and corrosion performance due to theformation of Zn—Y—Nd—Zr coarse intermetallics. Furthermore, the alloysof the instant invention do not contain rare earth elements with lowsolubility in solid magnesium such as Ce and La. The presence of thoseelements results in deterioration of mechanical properties, andparticularly of ductility, due to the formation of coarse intermetalliccompounds. An admixture of calcium in the alloys of the invention of upto 0.02% may improve oxidation resistance.

The Ca content is limited to 0.02% because higher Ca content leads toincreased micro-porosity and embrittlement of the alloys.

The alloys of instant invention also do not contain heavy rare earthelements with atomic number higher than 60, they would increase thealloy price without remarkably improving the alloy performance.

Surprisingly simple alloys of the invention are suitable for HPDC andother applications, while exhibiting superior casting properties, highstrength, high creep resistance, high corrosion resistance, and thearticles manufactured from the alloys show superior performance at hightemperatures.

The magnesium alloys according to the invention were examined along withcomparative alloys. The results show that the new alloys exhibit betteroxidation resistance and fluidity, as well as lower susceptibility todie sticking than comparative alloys. Neither burning nor oxidation wasobserved on the surface of ingots made of alloys according to thisinvention. In contrast, the preparation of comparative alloys wassometimes accompanied by significant oxidation and undesirable losses ofalloying elements. The alloys according to the invention reached between96 and 100% on the relative castability scale, when evaluating oxidationresistance, fluidity, and susceptibility to die sticking (see Examplesbelow), in comparison with 73 and 83% of comparative alloys whosecomposition differed more or less from the composition of the invention.

Part of the ingots of both the new and the comparative alloys were thenremelted and high pressure die cast to produce different specimens fortesting and examination. Other ingots were remelted, grain refined andsand cast into different specimens for testing. Tensile Yield Strength(TYS), Ultimate Tensile Strength (UTS), percent elongation (% E),compressive strength (CYS) and different creep characteristics such asCreep Strength, Creep Rupture Strength, and Minimum Creep Rate were thendetermined. Corrosion behavior was evaluated by the GM 9540 cyclic test.The alloys according to the invention surpassed the comparative alloysin creep stress to rupture, creep strength, and corrosion resistance.They also exhibit better combination of strength and ductility,characterized by elongation values, than comparative alloys.

The alloys according to the invention are very suitable for HPDC; it wasfound that they develop excellent properties after direct T5 aging at150-250° C. for 1-10 h, preferably at 175-225° C. for 1-6 h. As forwrought alloys, it was found that the alloys according to the inventionachieve very good properties after direct aging at 200-250° C. for 1-16h. It was found that the alloys according to the invention also provideexcellent mechanical properties on sand casting after full T6 heattreatment; particularly, good results were obtained when the heattreatment comprised solid solution heat treatment at 520-560° C. for 1to 16 hrs, followed by cooling in a quenching media and by subsequentaging at 200-270° C. for 1 to 16 h, preferably after solid solutiontreatment at 535-545° C. for 3 to 5 hrs, followed by cooling in aquenching media and by subsequent aging at 225-250° C. for 3 to 6 h.

The invention will be further described and illustrated in the followingexamples.

EXAMPLES

The alloys of the present invention were prepared in 150 l crucible madeof low carbon steel. The mixture of CO₂+0.5% HFC134a was used as aprotective atmosphere. The raw materials used were as follows:

Magnesium (Mg)—pure magnesium, grade 9980A, containing at least 99.8%Mg.

Neodymium (Nd)—commercially pure Nd (less than 0.5% impurities).

Zirconium (Zr)—Zr95 Tablets, containing at least 95% Zr.

Yttrium (Y)—commercially pure Y (less than 1% impurities).

Calcium (Ca)—pure Ca (less than 0.1% impurities).

Contrary to the alloying procedure described in CN1752251, whereintermediate Mg—Nd, Mg—Ca and Mg— Zr master alloys were used, the alloysof the present invention have been prepared using pure Nd and pure Cathat significantly simplifies the process, reduces its duration andmarkedly lowers the alloy cost. Neodymium and zirconium were addedtypically at 770-780° C. with intensive stirring of the melt. Afteraddition of zirconium, the melt was held for 20-40 minutes to allow ironto settle. Yttrium was added after the iron settling, without intensivestirring, to prevent the formation of Y—Fe intermetallic compounds,which leads to excessive loss of yttrium. A strict temperature controlwas provided during the alloying in order to insure that the melttemperature will not increase above 785° C., thus preventing anexcessive contamination by iron from the crucible walls, and to ensurethat the temperature will not decrease below 765° C., thus preventing anexcessive loss of zirconium. Calcium was added prior to settling. Afterobtaining the required compositions, the alloys were held for 30-60minutes for homogenization, and settling of iron and non-metallicinclusions, and then they were cast into the 15 kg ingots. The castingwas performed with gas protection of the molten metal duringsolidification in the molds by CO₂+0.5% HFC134a mixture. The die castingtrials were carried out using an IDRA OL-320 cold chamber die castingmachine with a 345 ton locking force.

The castability was evaluated based on observed fluidity, oxidationresistance and die sticking or soldering. The casting temperature was710° C. Each of the properties (fluidity, oxidation resistance, diesticking) was evaluated by assigning from 0 to 10 points on a relativescale, the higher the better (see Table 2). The sum of the points for analloy divided by 30 and multiplied by 100 provides “castabilitycoefficient”, a relative assessment having a value between 0 and 100%,which characterizes the overall suitability of an alloy for die casting.The alloys according to the invention had castability coefficientbetween 96 and 100%, while comparative examples, even if differing onlyslightly from the new alloys of the invention, had castabilitycoefficient between 73 and 83%.

Tensile and compression testing at ambient and elevated temperatureswere performed using an Instron 4483 machine. Tensile Yield Strength(TYS), Ultimate Tensile Strength (UTS), percent elongation (% E) andCompression Yield Strength (CYS) were determined. The SATEC Model M-3machine was used for creep testing. Creep tests were performed at 200°C. and 250° C. for 200 h or until rupture under various stresses. Creepresistance was estimated by measuring rupture strength and creepstrength. Creep strength is usually defined as the stress, which isrequired to produce a certain amount of creep at a specific time andtemperature. It is a common practice to report creep strength as thestress, which produces 0.2% creep strain at a given temperature for 100hours. This parameter is used by design engineers for evaluating theload-carrying ability of a material for limited creep strain inprolonged time periods. Creep rupture stress is the stress resulting inspecimens rupture at a selected testing temperature for a certain time,usually 100 h. In addition, minimum creep rate at a steady state (MCR)was used to evaluate creep performance.

Corrosion behavior was evaluated as per GM9540 cyclic test for 40 days(FIG. 9). The test procedure includes three main stages, combining bothwet-dry transition and short sprays of light electrolyte solution. Inthis test a gradual increase of temperature is applied during the cycle.The die cast plates with dimensions of 140×100×3 mm were used. Theplates were degreased in acetone and weighed prior to the test. Fivereplicates of each alloy were tested. At the end of the test thecorrosion products were stripped in a chromic acid solution (180 g CrO₃per liter solution) at 80° C. about three minutes and the weight losswas determined. The weight loss was used to determine the averagecorrosion rate in mpy (milli-inch per year).

Tables 1 to 4 illustrate chemical compositions, castability parameters,and properties of alloys for HPDC according to the invention and ofcomparative alloys. The new alloys of the invention demonstratesignificantly better die castability evaluated by tendency to oxidation,fluidity and susceptibility to die sticking (Table 2), reflected by acastability coefficient of minimally 96%. As can be seen from Table 3,the new alloys are superior in tensile yield strength (TYS) andcompressive yield strength(CYS) over the comparative alloys at bothambient and elevated temperatures. The same is true for UTS values. Forexample, TYS values of the new alloys according to the invention at 200°C. are 150 MPa or more, usually 153 MPa or more, whereas the comparativealloys have lower values. Furthermore, new alloys exhibit much bettercombination of strength and elongation than comparative alloys.Corrosion resistance of the new alloys determined under GM 9540 cyclictest conditions (FIG. 9) also surpasses that property of the comparativealloys; the corrosion rate of the new alloys is less than 2.9 mpy,usually less than 2.7 mpy, such as 2.65 mpy or less (Tab. 3). Inaddition, new alloys also exhibit excellent creep resistance in thetemperature range 200-250° C., outperforming the comparative examples(Table 4). The creep rupture strength of the new alloys for HPDC istypically about 200 MPa or more at 200° C., and about 105 MPa or more at250° C. The MCR values of the new alloys are 1.5×10⁻¹⁰/s or less at 200°C. and 100 MPa, usually 1.0×10⁻¹⁰/s or less; the comparative alloys havehigher values even if differing only slightly in composition from thenew alloys (Tab. 4).

The excellent combination of these properties along with lowsusceptibility to hot tearing makes the alloys of the instant inventionthe most promising candidates for high pressure die casting of movingparts serving at high temperatures of 200-250° C. where low momentinertia and correspondingly low vibration are required.

Tables 5-6 demonstrate chemical compositions and properties of alloysfor sand casting according to the invention and of comparative alloyssubjected to full T6 heat treatment. The alloys of the instant inventionexhibit superior combination of TYS and Elongation in comparison withcomparative examples. The compressive strength of new alloys is alsohigher both at ambient and elevated temperatures. Furthermore a greatadvantage of the alloys of this invention is that they combine excellentmechanical properties with outstanding corrosion resistance whichoutperforms corrosion resistance of comparative alloys.

Tables 7-8 illustrate chemical composition and mechanical properties offorged alloys of instant invention. The alloys of present inventions andcomparative alloys were direct chilled cast, homogenized, forged and T5heat treated. The forged alloys of the present invention exhibit higherTYS and UTS values than comparative alloys at both ambient temperatureand 200° C. It is important that they the alloys according to theinvention have also superior elongation and significantly highercompressive yield strength.

While this invention has been described in terms of some specificexamples, many modifications and variations are possible. It istherefore understood that, within the scope of the appended claims, theinvention may be realized otherwise than as specifically described.

1. A lightweight alloy for high-temperature applications, consisting ofi) at least 94.8 wt % magnesium, ii) 2.5 to 4.6 wt % neodymium, iii)0.05 to 0.40 wt % yttrium, iv) 0.03 to 0.65 wt. % zirconium, and v)incidental impurities.
 2. An alloy according to claim 1, furthercontaining up to 0.02 wt % calcium.
 3. An alloy according to claim 1,comprising essentially no heavy rare earth (HRE) elements with theatomic number from 61 to
 71. 4. An alloy according to claim 1,comprising essentially no cerium, lanthanum, and praseodymium.
 5. Analloy according to claim 1, comprising essentially no zinc.
 6. An alloyaccording to claim 1, wherein the total content of Nd and Y is higherthan 4.3 wt %.
 7. A lightweight alloy according to claim 1, forprolonged operation at a temperature of up to 200° C.
 8. An alloyaccording to claim 1, usable for a process selected from the groupconsisting of high pressure die casting (HPDC), sand casting, investmentcasting, and permanent mold casting.
 9. An alloy according to claim 1,usable for a process comprising either twin roll casting with subsequentrolling, or direct chill casting with subsequent forging, extrusion orrolling.
 10. A lightweight alloy for high-temperature applicationsaccording to claim 8 usable for HPDC.
 11. An alloy according to claim10, which contains 2.8 to 4.3 wt % Nd, 0.06 to 0.25 wt % Y, 0.05 to 0.4wt % Zr, and 0.0 to 0.02 wt % Ca.
 12. An alloy according to claim 10,exhibiting a castability of at least 96%, when measured in relativeunits characterizing oxidation resistance, fluidity, and dies sticking.13. An alloy according to claim 10, exhibiting a tensile yield strength(TYS) at 200° C. of at least 153 MPa.
 14. An alloy according to claim10, exhibiting a compression yield strength (CYS) at 200° C. of at least152 MPa.
 15. An alloy according to claim 10, exhibiting a minimum creeprate (MCR) of not more than 1.5×10⁻¹⁰/s at 200° C. under stress of 100MPa.
 16. An alloy according to claim 10, exhibiting a corrosion rateunder GM 9540 of not more than 2.7 mpy.
 17. An article cast of an alloyaccording to claim 10, exhibiting a superior combination of strength andductility after T5 treatment which includes direct aging at 150-250° C.for 1-10 h.
 18. An article according to claim 17, exhibiting a superiorcombination of strength and ductility after T5 treatment which includesdirect aging at 175-225° C. for 1-6 h.
 19. An alloy according to claim 8suitable for sand casting, investment casting, permanent mold casting,and low pressure modifications thereof, containing 2.7 to 3.4 wt % Nd,0.15 to 0.40 wt % Y, 0.3 to 0.6 wt % Zr, and 0.0 to 0.02 wt % Ca.
 20. Anarticle cast of an alloy according to claim 19, exhibiting a superiorcombination of performance properties after full T6 heat treatmentcomprising solid solution heat treatment at 520-560° C. for 1 to 16 hrs,followed by cooling in a quenching medium and by subsequent aging at200-270° C. for 1 to 16 h.
 21. An article according to claim 20,exhibiting a superior combination of performance properties after fullT6 heat treatment comprising solid solution heat treatment at 535-545°C. for 3 to 5 hrs, followed by cooling in a quenching medium and bysubsequent aging at 225-250° C. for 3 to 6 h.
 22. An alloy according toclaim 9, suitable for forging, extrusion, and rolling, which contains2.8 to 3.8 wt % Nd, 0.20 to 0.40 wt % Y, 0.35 to 0.60 wt % Zr, and0.0-0.02 wt % Ca.
 23. An article cast of an alloy according to claim 22,exhibiting a superior combination of performance properties after T5heat treatment comprising aging at 200-250° C. for 1 to 16 h.